The present disclosure describes systems and techniques relating to processing information before spatially diverse transmission.
Mobile phones, laptops, personal digital assistants (PDAs), base stations and other systems and devices can wirelessly transmit and receive data. Such systems and devices have used orthogonal frequency division multiplexing (OFDM) transmission schemes, such as those defined in the Institute of Electrical and Electronics Engineers (IEEE) 802 wireless communications standards. The IEEE 802 standards include IEEE 802.11, 802.11a, 802.11b, 802.11g, 802.11n, 802.16, and 802.20. In an OFDM system, in particular, a data stream is split into multiple substreams, each of which is sent over a different subcarrier frequency (also referred to as a tone or frequency tone).
Some wireless communication systems use a single-in-single-out (SISO) transmission approach, where both the transmitter and the receiver use a single antenna. Other wireless communication systems use a multiple-in-multiple-out (MIMO) transmission approach, where multiple spatially separated transmit antennas and multiple spatially separated receive antennas are used to improve data rates, link quality or both. This is known as antenna diversity, spatial diversity or spatially diverse transmission.
In addition to spatial diversity, many wireless communication systems also use time diversity and frequency diversity to improve system performance. Data streams can be encoded using channel encoders of different rates and with different amounts of redundancies, and the encoded data streams can be interleaved to separate adjacent coded bits. At the receiver, bit-streams that have been corrupted during transmission over the wireless channel are combined using error correction techniques to reconstruct the original information in the data bit-streams.
OFDM systems can be implemented as SISO or MIMO communication systems and can provide various advantages, including a relatively simple receiver architecture and potential performance improvements through appropriate coding across OFDM tones. For example, OFDM MIMO systems have been designed to interleave encoded bits using a separate block interleaver for each spatial stream, with a block size corresponding to the number of coded bits in a single OFDM symbol. Each interleaver effects a two-step permutation, where the first permutation ensures that adjacent coded bits are mapped onto nonadjacent subcarriers, and the second permutation ensures that coded bits are mapped alternately onto less and more significant bits of the constellation, thereby avoiding long runs of low reliability bits. In addition, various proposed interleavers can apply different data rotations to each spatial stream, before mapping to the subcarriers of an OFDM symbol, to improve the transmission.